An infrared sensor for detecting an electromagnetic wave (or an infrared ray) with a wavelength of 3 μm to 10 μm has been used as a heat sensing sensor in crime prevention, measuring, remote sensing and various other fields of applications. An infrared image sensor, in which such sensors are arranged as a two-dimensional array, can obtain an even greater amount of information as a thermal image, and has been used extensively in those fields of applications.
Infrared sensors are roughly classified into quantum sensors and thermal sensors. A quantum sensor is a sensor that is made of compound semiconductors and that operates by utilizing the band-to-band transition. Such a quantum sensor has higher sensitivity and higher response speed than a thermal sensor but operates at relatively low temperatures, thus requiring a cooling mechanism to maintain such low temperatures. That is why it is difficult to reduce the size or manufacturing cost of such a quantum sensor and it is not easy to apply it to cars, crime prevention tools and various other consumer electronic products.
On the other hand, a thermal sensor has lower sensitivity than a quantum sensor but needs no cooling mechanism to maintain low temperatures. For that reason, it is relatively easy to reduce the size and price of such a sensor, and therefore, it has been used extensively in various consumer electronic products. The thermal sensors include thermopile types, bolometer types and pyroelectric types.
A thermopile type includes a portion in which a lot of thermocouples are connected in series together as a thermal sensing portion. A bolometer type includes a resistor that is made of a material, of which the electrical resistance has significant temperature dependence. By detecting a variation in the amount of current flowing through that resistor, the bolometer type sensor can measure the temperature. Meanwhile, a pyroelectric type detects charge to be produced on the surface of a tourmaline crystal, for example, as the temperature varies, thereby sensing the temperature variation.
A thermal sensor of any of these types has a heat insulation structure to prevent the heat from escaping from its infrared sensing portion, thereby maintaining the sensitivity of the sensor reasonably high. An exemplary heat insulation structure for such an infrared sensor is disclosed in Patent Document No. 1, for example.
Hereinafter, the structure of a thermal infrared sensor as disclosed in Patent Document No. 1 will be described with reference to FIG. 8, in which FIG. 8(a) is a plan view illustrating main portions of this infrared sensor and FIG. 8(b) is a cross-sectional view of the sensor as viewed on the plane 8b-8b. 
The infrared sensor shown in FIG. 8 includes a substrate 240 of silicon, for example, and a photosensitive section 241 that is supported on the substrate 240. The photosensitive section 241 includes a bolometer 242, of which the electrical resistance has temperature dependence, and an interconnect 243 for measuring the electrical resistance of the bolometer 242. And the photosensitive section 241 functions as a heat sensing section for the infrared sensor. The interconnect 243 may be made of a metal such as aluminum.
On the upper surface of the substrate 240 that is opposed to the bolometer 242, a recess has been cut so as to leave a gap between the photosensitive section 241 and the substrate 240. Such a recess may be formed by selectively removing a predetermined region of the substrate 240 by either a wet etching process or a dry etching process.
The photosensitive section 241 contacts with the substrate 240 at contact portions 245. Both ends 244 of the interconnect 243 run over the contact portions 245 and are connected to a read circuit (not shown).
Hereinafter, it will be described how the infrared sensor shown in FIG. 8 operates.
When the photosensitive section 241 absorbs an infrared ray, the temperature at the bolometer 242 rises. As a result of the rise in temperature, the resistance of the bolometer 242 changes. In such a state, current is supplied to the bolometer 242 through the interconnect 243 and a variation in voltage, caused by the change of resistance, is detected. And based on the magnitude of this voltage variation, the energy of the infrared ray that has been incident on the photosensitive section 241 can be calculated.
The photosensitive section 241 preferably has a structure that can prevent the thermal energy, produced upon the exposure to the infrared ray, from escaping to the outside. In the example illustrated in FIG. 8, the area of contact between the body of the photosensitive section 241 and the substrate 240 is minimized to increase the heat insulation property. Also, the portions including both ends 244 of the interconnect 243 are elongated portions extending from the body of the photosensitive section 241 to reduce the conduction of the heat to the substrate 240. Thus, according to the method of Patent Document No. 1, by shaping the connecting portions between the photosensitive section 241 and the substrate 240 as elongate as possible, the heat insulation property between the photosensitive section 241 and the substrate 240 is improved. As a result, the magnitude of the variation in the temperature of the photosensitive section responsive to an incident infrared ray increases, thus increasing the amplitude of the signal to detect the infrared ray.
Another heat insulation structure for an infrared sensor is disclosed in Patent Document No. 2.
Hereinafter, the structure of the thermal infrared sensor disclosed in Patent Document No. 2 will be described with reference to FIG. 9.
The infrared sensor shown in FIG. 9 includes a lower substrate 110, an upper substrate (photosensitive section) 120, posts 210, lower electrodes 220, a reflective layer 230, and signal legs 200. A bolometer (not shown) is provided for the upper substrate 120.
When the upper substrate 120 absorbs an infrared ray, its bolometer comes to have an increased temperature and a varied resistance value. At this point in time, current is supplied to the bolometer 82 by way of the signal legs 200 with a metallic layer, thereby sensing a variation in voltage that has been caused due to the variation in resistance. And based on the magnitude of this voltage variation, the energy of the infrared ray that has been incident on the upper substrate 120 can be calculated.
The posts 210 are made of an insulator and perform the function of supporting the upper substrate 120 on the lower substrate 110. A cavity or a gap is left between the lower and upper substrates 110 and 120, thereby thermally insulating them from each other. The lower electrodes 220 produce electrostatic force with respect to the signal legs 200, thereby changing the positions of the signal legs 200. In this manner, the signal legs 200 and the upper substrate 120 can alternately have an in-contact state and an out-of-contact state.
When the signal legs 200 and the upper substrate 120 are out of contact with each other, the upper and lower substrates 120 and 110 are connected together with just the posts 210 that are made of an insulator. As a result, the heat insulation property between the upper and lower substrates 120 and 110 improves. Consequently, the rise in the temperature of the upper substrate 120 responsive to the incident infrared ray can be increased. By bringing the signal legs 200 and the upper substrate 120 into contact with each other after they have been out of contact with each other for a certain period of time, current is supplied to the bolometer and the quantity of the infrared radiation that has been incident on the upper substrate 120 is detected.
In such an infrared sensor, the magnitude of variation in the temperature of the photosensitive section responsive to the incident infrared ray increases, and therefore, the magnitude of variation in the resistance of the bolometer (i.e., the level of the signal to detect the infrared ray) can be increased, too.
As can be seen, an infrared sensor is required to have an increased magnitude of variation in temperature in response to an incident infrared ray, and eventually exhibit higher infrared sensitivity, by improving its heat insulation property.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-106896        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2005-181308        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2005-11795        